The research performed has consisted of an analysis of the mechanisms involved in establishment, maintenance and termination of the tumor dormant state which follows L5178Y lymphoma cell sinecomitant immunication and challenge of DBA/2 mice. We found that subcutaneous implantation of L5178Y cells into DBA/2 mice followed in 10 days by nodule excision protected 100% of mice from the rapid outgrowth of an intraperitoneal challenge of L5178Y cells given 7 days later. All sinecomitant immunized and challenged mice remained clinically normal for 60 to 360 days before succumbing to fapid tumor outgrowth. The numbers of L5178Y cells in the peritoneal cavitx increased rapidly for 4 days after challenge and then declined to low but detectable levels which persisted throughout the clinically normal period. A cell-mediated cytolytic (CMC) response to L5178Y cells was elicited in the peritoneal cell population reaching peak levels 4 days after challenge and then declining to lower but detectable levels which were maintained for much of the tumor dormant period. CMC activity became undetectable at a time preceding the onset of tumor cell emergence. Significant tumor specific humoral responses were absent throughout the period of tumor dormancy. The tumor cells which emerged following the prolonged dormant period were progeny of the original L5178Y cell inoculum and not of the newly transformed host cell. These emergent cells were less susceptible to tumor specific CMC than the original L5178Y cells.